PREFACEThe purpose of this serial publication is to provide authoritative reviews of progress in crop science, soil science, and agronomic practice.If there is a single theme, it is the soil-plant relationship. Most of thearticles in this volume exemplify the theme. One indeed goes further andin an interesting way brings in consideration of the role of the animal inthe cycling sequence.Great strides have been made in the improvement of crop plantsthrough genetic recombination. The acre yield of many crops has beensubstantially increased by developing varieties better adapted to theenvironment, but there are still potential gains to be made, both in yieldand quality, if the limiting biochemical processes can be identified. Intheir chapter on this topic, Hageman and colleagues discuss the natureof the opportunities thus presented. Physiological factors under geneticcontrol are dealt with by Quinby in reviewing the maturity genes insorghum, a crop the geographic range of which has been considerablyextended in recent years.All plant breeders are properly concerned with the preservation ofseed stocks and the maintenance of gene pools. The unique facilityerected by the U. S. Department of Agriculture for this purpose isdescribed by its Director, Edwin James.More applied topics are treated in a chapter on the growth andnutrition of flu-cured tobacco by McCants and Woltz and in one on thesoil and nutritional requirements of an important Australian tree crop,Pinus radiata, by Raupach.In another article Barley and Greacen, Australian authors, take up inan analytical mood one of the oldest problems of plant growth, thepenetration of roots through the soil and the emergence of seedlingshoots, as affected by the mechanical stress of the environment. Recentdevelopments in our understanding of soil forms of phosphorus andphosphorus transformation in soils are presented in a scholarly reviewby Sigurd Larsen. This is another old topic that is steadily reshapedbecause of continuing attention to the essential and dynamic role playedby this element in plant growth.The eight chapters in this volume are indicative of the diversity andvitality of researches in soil and crop science that lead to improvementsin practice and to the benefit of man.A. G. NORMANAnn Arbor, MichiganJune,1967vii

Man has been aware of the importance of the mechanical propertiesof the soil since agriculture began. He cultivated the soil when it wasmoist because it was then easier to deform. He was well aware that theemergence of his seeded crops could be hindered by a hard crust.In the late nineteenth century the work of Darwin and others stimulated considerable interest in the adaptation of plants to their mechanicalenvironment. In the same period fundamental discoveries were made1

2

K. P. BARLEY AND E. L. GREACEN

about the chemistry of the nutrition of plants, and after the turn of thecentury interest was centered on this subject. Later the center of scientificinterest shifted to physical studies of the water, air and heat relations ofthe plant. Although it was realized that the mechancial properties ofthe soil could sometimes be of great importance, the slow developmentof soil mechanics hindered further analysis of the influence of this soilfactor on plant growth. The fact that soil mechanics has been the domainof the engineer has been a further handicap in applying the subject toagronomic problems. Frequently a practical empirical solution has beenobtained by the engineer, which, although it solves a construction problem, may do little to explain the processes involved.Recent discoveries in soil and in plant mechanics promise betterunderstanding of the way in which mechanical properties of the soilinfluence plant growth. In this review we intend to discuss chiefly thepenetration of the soil by roots and emerging shoots. We remind thereader that mechanical factors also operate in other processes of considerable agronomic interest; a few examples are the burial of fruitingorgans by certain crop and pasture legumes, the radial enlargement ofedible underground organs, and the uprooting of crops or trees.Although roots and shoots may grow mainly through existing voidsin openly structured soils, whenever these organs penetrate peds orhorizons that lack wide pores they have to deform the soil. The soil resistsdeformation, and the growing organ is stressed mechanically by the reaction of the soil to the force that the organ exerts. It is well known thatstrongly cemented or indurated horizons exclude roots, and that strongcrusts prevent emergence (Lutz, 1952); but in this review we aim toassess the importance of mechanical resistance in ordinary soils.We define mechanical resistance as the reaction of the soil to forcesexerted by the growing plant. As the intercellular or “pore” space withinplant organs is normally highly permeable to both air and water, differences in pore fluid pressure cannot be long sustained across a plant-soilboundary. Large gradients, of course, may exist within the soil itself. Itfollows that, except in transient states, we are concerned with the reaction on the plant of the solid phase of the soil.II.

Types

of Deformation Produced by Plants

The theory of soil mechanics, and the methods used to measure themechanical properties of soils have been developed almost exclusivelyfor engineering applications. The foundations engineer is concerned withthe maximum force that a soil can withstand without undergoing alarge displacement, that is, with the ultimate strength of the soil; whereasthe biologist wants to know the force that will deform a soil sufficiently

MECHANICAL RESISTANCE OF SOIL

3

to allow a root or shoot to grow. Differences in scale are also important:the engineer deals with stresses acting over areas of square meters andcan employ a statistical concept of stress; in plant studies we are concerned with areas of the order of one square millimeter, and the plantorgan is often commensurate in size with the structural or mechanicalelements of the soil.A. TENSILEFAILUREOne manifestation of tensile failure is the rupturing of soil crusts byemerging shoots. An appropriate measure of the strength of crust materials being deformed in this way is the modulus of rupture (Carnes,1934). The force required to rupture the crust depends on the dimensionsof the ruptured plates, and emergence should be related to this forcerather than to the modulus itself. Arndt (1965) points out that ruptureof the surface crust can be followed by jamming of the broken plates ofsoil (Fig. l a ) , This increases the force required for emergence.

FIG. 1. ( a ) Examples of soil deformation by emerging seedlings. The surfaceseal has cracked naturally, or been ruptured by the plant, with the plates subse(a' + z')"'. ( b ) Shear failurequently jamming. Jamming occurs when a + dl d'in the form of an inverted cone. (From Arndt, 1965.)

+ <

Roots can also rupture soils by tensile failure. Barley et al. (1965)observed that radicles of peas, Pisum sutivum L., 2 mm. in diameter,were able to split cores of compact loam (Fig. 2 ) . In contrast, the thinner(0.3 mm. diameter) radicles of wheat, Triticurn aestivurn L., formedchannels in cores of compact loam, but the bursting force was not greatenough to rupture the cores.Rupturing may involve either general or local tensile failure. When

a failure is general, by definition, it spreads to a soil boundary; in localfailure the tension cracks do not extend to the boundary but are accommodated by compression of the soil.B. SHEARFAILUREWITHOUT COMPRESSIONBesides failing under tension, soils also fail under shearing stressesimposed by plant organs. Terzaghi (1943, p.119) describes generalshear failure in soils under shallow foundations. In Terzaghi’s model thesoil compresses little with increasing application of the load until acritical load is reached, when the soiI fails completely. Failure takesplace on a sliding surface described by a plane and a logarithmic spiral.The load that the soil will support depends on the strength parameters,(Terzaghi,apparent cohesion, c, and the angle of internal friction,1943).The kind of failure described by Terzaghi has been observed whenroots first penetrate saturated clay (Cockroft, unpublished data). Anexample of general shear failure caused by seedling emergence has beengiven by Arndt (1965) (Fig. l b ) ; the soil fails along the surface of aninverted cone having its apex at the top of the seedling,

+

C. SHEARFAILUREWITH COMPRESSIONIn unsaturated compressible soil much of the volume increase of thegrowing plant organ may be accommodated by compression, and the

MECHANICAL RESISTANCE OF SOIL

5

zone of shear failure in which the stresses are in “plastic equilibrium”(Terzaghi, 1943, p.23) may frequently fail to spread to a soil boundary.When this is so we speak of “local shear failure.” Examples of local shearfailure with compression caused by growing roots have been given byBarley (1954, 1963). Roots were shown to have compacted coarse textured media for a radial distance of several millimeters around the root.The volume of the cores in which the roots were grown remained constant. Shear, together with compression, is probably the most commonway in which growing plant organs deform ordinary, unsaturated soils.In saturated clay plant organs may form channels by consolidationtogether with shear failure. If the volume of the root is accommodatedwithout displacing the boundaries of the clay, as water and clay areonly slightly compressible, water must be either absorbed by the penetrating root or drained through an outer boundary of the clay. Thisprocess, by definition, involves consolidation ( Terzaghi, 1943, p.265) ,but, as a hole is being formed, shear failure must also occur.The process described above differs from one-dimensional consolidation as met in engineering practice. In one-dimensional consolidation theconsolidating axial stress, ul,and the resulting radial stress, us,are notin plastic equilibrium but are related by the expression u3 = K,u,, whereKO is the coefficient of earth pressure at rest. For medium-textured soilswith 9 = 40°,K Oz 0.5, and for clays with lower values of 9, K O variesfrom 0.6 to 1.0. When consolidation is accompanied by shear failure thetwo stresses are related by the coefficient of active earth pressure, K ,(Terzaghi, 1943, p.50); K , is as low as 0.2 for coarse-textured soils butcan approach 1.0 for clays.Ill.

Forces Required to Deform Soils

A. THEORY

1. Tensile FailureGeneral tensile failure of surface crusts is commonly treated in termsof elasticity theory. In the modulus of rupture test the force, F , requiredto rupture a slab of length a, width b, and thickness z, for single-centerpoint loading is given by

and for two-point loading at a / 3 and 2 a/3 by

6

K. P. BARLEY AND E. L. GREACEN

where up is the tensile strength of the soil. Analyses of tensile failure formore complicated configurations are available in the theory of elasticity( Timoshenko and Goodier, 1951) .The tensile rupture of bulky structures can also be described theoretically. Applying a spherical model, the zone of plastic equilibrium aroundthe base or point of a probe can be treated as a pressure bulb of radius R(see Section 111, A, 3 ) . The radial pressure at R, u ~will,burst a soil clodif the cross-sectional area of the structural element is such that tensileresistance is less than the force developed over the cross section of thepressure bulb. Whether a clod will fail in tension depends then on themagnitude of uR,the tensile strength of the soil uT,and on the size of theclod. If rupture occurs during radial enlargement rather than duringpenetration a cylindrical model should be used.Local radial cracks may develop either around individual roots or between adjacent root channels (Fig. 2 ) . Using either a spherical or cylindrical model, the tangential stress U t , which reaches a maximum at R,closely approaches the tensile strength of the soil. Where the plastic zonesof adjacent roots overlap v(Tt is increased, and local rupture is likely tooccur.2. Shear Failure without CompressionThe conventional description of forces acting on the base of a pileor probe (Terzaghi, 1943) shows that the bearing capacity qp of ashallow ( z = d ) foundation, of depth z and width d, failing in generalshear, is given byqp = cNc

+ P Z N ,+ pdN,

(3)

where c = apparent cohesion, p = bulk density, and N,, N,, Np =bearing capacity factors.The values of the bearing capacity factors depend only on the angleof internal friction, When saturated clays are distorted with negligibledrainage, the strength of the clay is not altered by an applied load sincethe load is carried by the pore water (see Section 111, C, 1). Shearstrength is then determined solely by c, and the soil is called a frictionless or = 0 soil. For circular shallow footings in saturated undrainedclay qpz 7.5 c. According to Terzaghi’s model qp increases continuouslywith x. This relation applies to rough probes entering saturated “undrained” clays, the requirement of the “undrained condition being meteither because the clay is so impermeable that it fails to consolidate, orbecause the rate of loading or penetration is so high that there is timefor only a negligible amount of consolidation.

+,

+

7

MECHANICAL RESISTANCE OF SOIL

With the exception of Terzaghi’s analysis for shallow foundationsthere are few analyses of general shear failure appropriate to biologicalproblems. The general shear failure that sometimes occurs above upwardacting penetrometers and seedling shoots is described in an analysisgiven by Balla (1961) for the anchorage of mushroomed pylons. Sohtions require the strength parameters c and + and the configuration ofthe system.3. Shem Failure with Compression

Where the soil does not behave as an ideal brittle or plastic material,but is compressed or consolidated during deformation, conventionaltheory is inadequate. For deep piles, z > 3d, a “plasticity” theorymodified from that of Terzaghi is usually employed (Meyerhof, 1951).Although Meyerhof‘s theory implicitly describes local shear failure, asshearing is depicted as occurring in a localized zone around the base ofthe pile, compression is not described explicitly. According to Meyerhof,for homogeneous saturated clay soils failing without drainage ( 4 = 0),qp attains a steady maximum at depth where qp = 10 c. Strictly, qpcannotattain a steady maximum in such materials, because the shearing zonewould have to extend to the full depth of the pile. But real clays areneither truly saturated nor homogeneous, and in practice the volume ofthe pile may often be accommodated locally, for example by displacementof the clay into cracks or fissures. In compressible soils, following Terzaghi (1943, p.130) an arbitrary reduction is made in c and 4. The bearing capacity factors have been elaborated by Meyerhof (1961) to includethe shape and roughness of the pile. His theory is useful for saturatedclays and for soils having 4 < 35” and failing with little compression.Since the factors become highly sensitive to changes in for values >35”,and as a large arbitrary reduction in + must be made in compressiblesoils, the theory lacks general utility.An analysis of the resistance offered to probes in compressible soilshas recently been made by Farrell and Greacen (1966). Followingearlier work on the distribution of stress in soil around holes (de Jongand Geertsma, 1953) , tunnels ( Terzaghi, 1943), and around piles(Nishida, 196l), they postulate the existence of two main zones of compression around the point of a penetrating probe: a zone of shearingfailure called the plastic zone, and outside this an elastic zone (see Fig.3 ) . Farrell and Greacen assume that the pressure on the base of a probeis equal to the pressure required to form a spherical cavity in the soil.This approach is not new. Previously Bishop et al. (1945) had used themodel of an expanding cavity in a study of indentation tests in copper.Ladanyi (1963) used a similar model to describe pile penetration into a

+

8

K. P. BARLEY AND E. L. GREACEN

saturated undrained clay, and Nishida ( 1961) calculated the pressurerequired to expand a cylindrical cavity in the soil.The new contribution of Farrell and Greacen is their treatment ofthe compressibility of the soil. The analyses of Bishop et al. and Ladanyiconcerned incompressible material. Nishida assumed that the volumeu2~,)/3,change was determined by the mean principal stress, ( u1where the subscripts refer to the principal stresses. Vanden Berg et al.(1958) also used the mean principal stress, but Sohne (1958) used themajor principal stress. Farrell and Greacen largely overcome this ambiguity by using an experimental curve for compression accompanying

+ +

PRINCIPAL STRESS

U,. (bar)

(a)FIG. 3. Compression curves ( a ) associated with the zones of compression I-IV( b ) around the point of a penetrometer in compressible soil: I , e = emin,11, failurezone, I l l , rebound zone, and lV, elastic zone.

shear failure. In the plastic zone there are three distinct subzones ofcompression (Fig. 3 ) : I, where the soil is compressed to the minimumvoid ratio’ emin;11, where the soil undergoing failure behaves as amaterial being compressed for the first time; 111, a rebound zone wherethe soil behaves as an “overconsolidated”material (see Section 111, C, 2 ) .After equating the change in volume of voids in the various zoneswith the volume of the probe, Farrell and Greacen find the radius of theplastic zone, R, and, knowing this, the pressure qp on the base of a smooth(frictionless) cylindrical probe. The theoretical value of qp for a smooth

probe can be checked experimentally by rotating a real probe to dissipatefriction in the tangential direction. When this was done Farrell andGreacen found good agreement between theoretical and measured valuesof qpin a range of finely structured soils.Ordinarily, friction is mobilized both at the base (“point” friction)and along the curved cylindrical barrel (“skin” friction) of a probe.Point friction is appreciable for metal probes in soil. For example, itincreases the value of qp for real as opposed to smooth probes by as muchas 40 percent when the angle of soil-metal friction, 8, = 23” (Farrell andGreacen, 1966). When the additional expression for point friction is incorporated, the theory of Farrell and Greacen may be used to predict qpfor real, nonrotated probes. The agreement obtained with measured values for steel probes in three soils is shown in Table I (see p. 15).It seems likely that qP for root tips is less than qp for steel probes, asan estimate of the friction angle, 6, for the interface between root tips< Ssteel-soil (see Sectionand sand (Barley, 1962) suggests that SrOOt-SO,l111, A, 4). However no data are available for the immediately relevantinterface between root cap and soil. It is possible that the well knownsecretion of mucigel by cells of the root cap is a means of reducing 6.Recently Farrell and Greacen have extended their theoretical analysisto include cylindrical enlargement. Surprisingly, when 4 is large, say40”,the pressure required for the radial enlargement of a cylindricalcavity is only one-fifth of that required for a spherical cavity. The difference between the two pressures decreases with decreasing values of 4.Clearly, the shape of a penetrating object may have a large influence onthe resistance encountered in high 4 soils. The cylindrical model is likelyto be more appropriate when the tip is acutely tapered.

4 . Skin FrictionIn foundations-engineering the total axial pressure, q, that a pile canwithstand, or, in other words, the axial pressure that has to be applied topenetrate the soil, is termed the bearing capacity and is given by

(4)where qp = point pressure; qf = axial pressure needed to overcome skinfriction on the curved cylindrical wall of the pile.Usually adhesion and skin friction are lumped together and estimatedempirically. For rough piles in “undrained clay, skin friction per unitcurved wall area may be s e t equal to c, and the bearing load due to skinfriction Qf = %JOzcrdz, where r is the radius of the pile. For drainedconditions Eide et al. (1961) represent the radial load on the shaft asKuz, where a, is the effective axial pressure and K is a coefficient of earthP=

QP

+,Qf

10

K. P. BARLEY AND E. L. GREACEN

pressure. Then, Qr = 2 ~ / o x K tan~ Z r6 dx. For rough piles 6 may be setequal to 4.Little is known about the skin friction and adhesion at the interfacebetween plant organs and the soil. One value of 8, reported for a root“soil” interface, pertains to the root tip of maize and a moistened plateof cemented sand (Barley, 1962). This value of 6 was obtained directlyby the following method: first, root tips with a flattened “face” wereobtained by pressing roots against the plate as they grew. The tip wasthen severed and secured to a slider with small barbs. Finally, the flatface of the root tip was forced against a portion of the plate mounted ona friction trolley. The measured value of 8 was 17”.Recently Barley and Stolzy (1966) used as a crude measure of Qf theforce required to pull out a penetrating root tip. For peas (Pisumsativum L.) in a moist loam Q, was one-fifth of the total resistance topenetration Q. The pulling method is used in engineering to measure Q,for piles, and it is usefuI in clays. In sands the radial pressure on the pileis relieved by the upward pull and friction is underestimated.In contrast to piles, where the whole buried length is pushed throughthe soil and meets with frictional resistance, in the root only the shortlength from the cap to the proximal limit of the zone of elongation ispushed through the soil. Friction occurs behind the zone of elongation,but it is mobilized as anchorage to assist penetration, For emerging shootsthe location of the zone of elongation relative to the apex differs widelybetween species (Leonhardt, 1915). In many plants an appreciable partof the shoot is pushed upward through the soil, and skin friction cannotbe safely neglected in any analysis of the resistance opposed to emergence.

B. THEASSESSMENTOF MECHANICALRESISTANCEEstimates of the mechanical resistance opposed to growth must bebased on knowledge of the type of deformation produced by the plantroot or shoot. The type of deformation determines not only the soilproperties to be measured, but also, as we shall see, the methods to beused in measurement.1 . Determinatwn. of Strength ParametersThe parameters that describe the strength of a soil failing by shearwith little or no compression are the classical strength parameters c and4. The relationship between these parameters and certain derived measures of strength is described diagrammaticaIIy in Fig. 4.For any particular normal load, un, acting on a plane of failure, c and 4 give the shearstrength, sn, according to the Coulomb equationsn = c

+

U~

tan

ip

(5)

MECHANICAL RESISTANCE OF SOIL

11

The Mohr circle for the unconfined compressive strength, uc, is shownin Fig. 4;it can be seen that uc depends on c and 4. Farrell et al. (1967)have shown that, at pore water pressures as high as -0.3 bar, compactloams behave as brittle materials, for which uc = Sor (Griffith, 1924).Where the sample is in the form of a core, either natural or remolded,

FIG.4. Mohr diagram for an unsaturated soil with the failure envelope describedby c and @, u1 and u3 are the principal stresses; in a triaxial test these are the axialand the radial stresses, respectively. The shear stress 7 = ( uI - u3)/2. Mohr circlesfor the compressive strength, uc, and the tensile strength, uT, are also shown.

can be measured indirectly by means of the so-called Brazilian test(Kirkham et al., 1959) or uC can be measured by an unconfined loadingtest. Both tests are performed in a compression test machine; in theBrazilian test the lateral load required to rupture the core in tension ismeasured, and, in the second, the axial load required to rupture the corein shear is measured.Rogowski (1964) has pointed out that the above methods measurebulk strength of the soil and that the bulk strength is usually limited bythe inter-aggregate strength. Rogowski suggests that intra-aggregatestrength may be more important in controlling root penetration, becausethe root may often penetrate by deforming the adjacent aggregates ratherthan an extensive zone. He proposes that aggregate density be measured,strength then being determined on cores of soil remolded and compactedto the measured density. However soil strength is known to depend onthe stress history of the soil, and there is no simple relation betweendensity and strength (Section 111, C, 2 ) . Rogowski also developed a techUT

12

K. P. BAFLEY AND

E. L GREACEN

nique for measuring the crushing strength of small ( 2 to 3 mm.) aggregates, by rupturing them in an unconfined compression test between twoplates. He postulates that roots encounter a resistance that depends onthe crushing strength of the aggregates. However, even if this is so, hisanalysis is unsatisfactory as it stands because it neglects deformationsthat precede and accompany failure of the aggregates.Rogowski's criticism of the measurement of bulk soil properties hardlyapplies when the deformation spreads over a zone that is large comparedwith the size of the aggregates, that is, in finely structured soil. In soilswhere the aggregates are commensurate in width with the plant organconcerned, Rogowski's approach may be profitable.The derived measures: modulus of rupture, the Brazilian test, thecompressive strength, and the crushing strength each give a single Mohrcircle on the strength diagram (Fig. 4 ) . Because of this any one of thesemeasures provides useful comparative data only where 4 is constant oralmost so. As mentioned in Section 111, A, 2, saturated, undrained claysbehave as if they were 4 = 0 materials. In unsaturated soils or in fullydrained clays 4 usually varies between 20" and 45" (Fountaine andBrown, 1959), not being greatly affected by changes in void ratio orpore water pressure. It should be noted, however, that occasionally muchlower values have been reported (Payne and Fountaine, 1952).A satisfactory characterization of strength for failure with little or nocompression is obtained by describing the failure envelope on a Mohrdiagram with one of the recognized techniques. The torsion shear box(Payne and Fountaine, 1952) or the direct shear box (Terzaghi andPeck, 1948) are often employed, the former being useful for small (25cc.) samples or peds. The most versatile method for soil cores is thetriaxial compression test, a comprehensive account of which is given byBishop and Henkel (1962).Where the deformation involves local shear failure with compression,analytical estimates of mechanical resistance require the strength parameters c and 4 together with a measured compressibility curve. The compressibility characteristics may be expressed as a Young's Modulus andas the gradients of the failure and rebound curves for compression withshear (see Section 111, A, 2). The parameters c and 4 and the compressibility characteristics are equally important in determining the resistanceto penetration. As Farrell and Greacen (1966) have shown they can bemeasured with sufficient accuracy by means of the triaxial cell,No general relation is to be expected between void ratio, e, and theresistance that soils offer to penetration, Q. When e>>e,,i, for a particular soil most of the volume change occurs in the zone of compressionwith failure; as e approaches eminthe rebound zone and the zone of